As is known in the art, as the frequency of FET (Field Effect Transistor) operation goes above 90 GHz toward THz range, both gate and channel lengths of FETs are reduced to sub-80 nm range towards 20 nm according to recent publications. At these small geometries, parasitic resistance, inductance, and capacitance of the FETs significantly affect the RF performance of the FET device, such as power, gain and efficiency. Most of attempts to increase the operating frequency of FET have focused on using smaller gate length and width, and narrow channel.
More particularly, a FET according to the PRIOR ART is shown in FIGS. 1A-1C. Here, a semi-insulating, highly resistive, substrate, such as SiC, has formed on the upper surface thereof a mesa shaped semiconductor structure, here for example a Group III-V structure, here, for example, a GaN structure. More particularly, III-V based structures such as GaN-based transistors use electrons formed between two different bandgap materials, for example, AlGaN and GaN. Formed in ohmic contact with source and drain regions of the upper surface of the mesa are source and drain electrodes, as shown. Disposed between the source and drain electrodes is a gate electrode in Schottky contact with an upper surface of the mesa (a gate region) disposed between the source and drain regions. The gate electrode is used to control a flow of carriers (holes and electrons) in an active region of the mesa though the active region (sometimes herein referred to as the gate channel region) between the source and drain regions. It is noted that the regions outside of the mesa area, called ‘off mesa area’. The off mesa area, as noted above, is semi-insulating highly resistive area. The Effective gate width (the active region) is the length of the gate electrode is the region closest to the source and drain regions (the gate channel region) and it is this gate channel region that contributes to the conduction of transistor. As the gate channel width (the distance between the source and drain) gets narrower to reduce the electron transfer time for high frequency operation, the contribution of the carriers in the gate channel region gets stronger. From the prior art it is noted that the total gate length extends beyond the gate channel length even though the most of carrier conduction occurs along the gate channel length; however, the portions of the gate electrode extending beyond the gate channel region generate parasitic gate resistance, inductance, and capacitance and thereby contribute negatively for high frequency operation.
Next, it is noted that the source electrode is disposed within opposing sidewalls of the mesa structure, the drain electrode is disposed within the opposing sidewalls in ohmic contact with a drain region of the mesa, and the gate electrode is disposed within opposing walls of the gate region of the mesa and that the mesa is rectangular shape. Further, it is noted that the source electrode has an inner edge extending between ends SOURCE EDGE 1, SOURCE is EDGE 2 (FIG. 1A) thereof proximate the gate electrode that extends along a direction parallel to the gate electrode; and, likewise the drain electrode has an inner edge extending between ends DRAIN EDGE 1, DRAIN EDGE 2 (FIG. 1) thereof proximate the gate electrode that extends along a direction parallel to the gate electrode. The lengths of the inner edges of the source and drain electrodes are equal. The gate electrode extends beyond the ends of SOURCE EDGE 1, SOURCE EDGE 2 of the inner edge of the source electrode and thus also beyond the ends of DRAIN EDGE 1, DRAIN EDGE 2 of the inner edge of the drain electrode. The active region (gate channel) extends between ends DRAIN EDGE 1 (or SOURCE EDGE 1) and DRAIN EDGE 2 (or SOURCE EDGE 2) (FIG. 1A). As noted above, the total gate electrode length extends beyond the active region (gate channel length), even though the most of carrier conduction occurs along the gate channel length, generates unwanted parasitic gate resistance, inductance, and capacitance and thereby contributes negatively for high frequency operation of the FET.